This invention relates to magnetic scaling devices, and is particularly directed to a magnetic scaling device in which a magnetic reading head provides signals corresponding to indicia recorded on a magnetic scale member to indicate accurately any relative displacement between the magnetic head and the magnetic scaling member. Such a magnetic scaling device finds favorable application, for example, in the control of automatic machine tools or industrial robots.
Magnetic scaling devices are well known. The construction and operation of a typical such device is disclosed in U.S. Pat. No. 3,597,749. Such magnetic scaling devices include a magnetic scale and one or more reading heads arranged so that indicia on the magnetic scale cause the magnetic reading head or heads to generate scale signals when the magnetic scale is moved past the reading heads. These scale signals can be used to derive an accurate indication of the linear displacement between the magnetic scale and the reading head.
Such devices basically employ an incremental measuring procedure. That is, the indicia on the magnetic scale correspond to incremental amounts of displacement. Thus, in order to ensure that the reading heads provide an accurate measurement, each of the indicia must be detected accurately, and no extraneous signals should be generated from the reading heads.
Unfortunately, the magnetic scaling devices are most often employed in an environment containing cutting oil, dust, small particles of magnetic material, or other contaminating elements, owing to the operation of the machine tool in connection with which the magnetic scaling device is used. As a result, the magnetic scale and the reading head can lose their accuracy in a short time unless the magnetic scaling device is adequately protected against these elements.
In order to accomplish this, magnetic scaling devices are often constructed to include a housing or casing having a small opening therein, provided with a dust seal, through which a member extends for moving either the head or the magnetic scale with respect to the housing. The other of the head and magnetic scale is then fixedly disposed on the interior of the housing.
However, even with such construction, the seal provided at the opening is not entirely adequate for protection from contaminants, and, if the magnetic scaling device is used for an extended period of time, water and dust will enter the opening in the casing, and interfere with the accurate operation of the device.
In these magnetic scaling devices, a reference position is determined by use of a reference magnetizing body, normally affixed within the housing, and a reference detecting element, which can be, for example, a Hall element, providing a reference signal when the detecting element is aligned with the reference magnetizing body. In order to ensure that the reference detector is sufficiently protected from cutting oil, metal dust, and other such elements, the reference magnetizing body and the reference detecting element are enclosed within the housing.
Several magnetic scaling devices have been previously proposed in which the magnetic scale, the reading head, the reference magnetizing body, and the reference detecting element are all disposed within a common housing in order to provide adequate protection against the above-mentioned impurities. However, the construction of such devices is usually complicated, and it is difficult, and sometimes impossible, to determine the specific locations of the magnetic scale and of the reference magnetizing body once the device is assembled. However, attempts to simplify the construction of such devices have resulted in devices lacking sufficient protection from these impurities.